Seoul’s Unification Ministry and the National Intelligence Service did not confirm the report but said they were looking into the issue.

The report said the escape took place after May 9, when Pyongyang wrapped up its rare ruling party congress, the first in 36 years.

Tough United Nations sanctions imposed on North Korea after its January nuclear test significantly curtailed the isolated state’s ability to earn hard currency, making the restaurants an even more important source of income than before.

There have been reports of staff not being paid, with restaurants pressured into increasing their regular remittances to Pyongyang.

The South Korean government estimates that Pyongyang rakes in around US$10 million every year from about 130 restaurants it operates – with mostly North Korean staff – in 12 countries, including neighbouring China.

Since the group defection in April, North Korea has claimed that the restaurant workers were tricked into defecting by South Korean spies, repeatedly running emotional interviews with their parents on state media.

Pyongyang also called on Seoul to repatriate its citizens and demanded a face-to-face meeting with the defectors and their family members, which the South flatly rejected.

North Korea is known to be strict about vetting and selecting people to work in its overseas restaurants, knowing they will inevitably be exposed to information about the outside world that they are mostly protected from in the North.